Active sonar antenna systems collect information about the direction of objects or sound sources as well as their range. In order to achieve angular resolution the antenna must have directional properties. A number of individual omnidirectional hydrographic transducers may be arrayed and operated to obtain the desired directivity.
In general a hydrophone receive array will receive maximum intensity from a steered direction, and will also receive side lobes of lesser intensity offset from the principal direction. The overall pattern is known as the ‘beam pattern’, and the antenna is generally operated using a process called ‘beam forming’ to achieve a desired pattern.
Beam forming consists in summing the individual sensor signals with appropriate delays and weights. If it is done in the frequency domain, or in the time domain with narrowband signals, no delays are needed, because they can be incorporated as phase within complex weights.
Beam forming is well understood and produces unambiguous results from simultaneous returns from the bottom, but suffers from a number of drawbacks. It is expensive since a large well populated array is required for good angular resolution. The sensors must also be packed close together to avoid aliasing. Considerable computational resources are required to process the large number of sensors.
Alternatively, interferometric methods may be used. Interferometry provides an alternative to beam forming. The technique consists in estimating the time delays between sensors, either by correlation, or timing the interval between zero crossings; the travel time gives the range and the difference in time of between the two sensors gives the angle. This technique provides very high resolution, the hardware is relatively simple and little processing is required, however there are a number of drawbacks. For instance, the technique needs careful calibration and is unable to resolve more than one source at each range without special fixes. The technique enjoys poor public image and is fragile. The results need intelligent filtering and interpretation.